1. Field of the Invention
The present invention relates to a device particularly suitable for the coextrusion of thermoplastics and especially of thermoplastics which are sensitive to thermal degradation.
2. Background of the Art
It is already known to use devices for the coextrusion of thermoplastics, especially to produce tubular articles or multi-layer films combining thermoplastics having different and/or complementary properties.
Thus, U.S. Pat. No. 4,047,868 describes a device for the coextrusion of tubular articles, which comprises an elongate central core and an external structure surrounding the core and defining, together with the latter, an extrusion channel extending, in the direction of the longitudinal axis of the device, to an outlet orifice, in which device the external structure consists of a series of successive elements each comprising a toroid feed chamber surrounding the extrusion channel and connected to a feed source of molten thermoplastic, with an extrusion passage connecting the periphery of the toroid feed chamber to the extrusion channel and with an adjustment ring, movable transversely relative to the longitudinal axis of the device, to provide control of the flux of molten thermoplastic flowing from the toroid feed chamber to the extrusion channel.
In the devices of this type, the adjustment ring generally acts only on a portion of the extrusion passage close to the extrusion channel, resulting in a--sometimes substantial--change in the convergence zone of the molten thermoplastic, thus creating a local potential source of instability of flow which can moreover cause thermal degradation of the processed polymer or polymers.
Moreover, the currently known coextrusion devices are so designed as to permit the production of coextruded articles comprising a defined number of layers and it is not possible to vary this number.
Finally, the currently known coextrusion devices are so designed as to permit either the extrusion of multi-layer tubular articles such as hoses or the extrusion of multi-layer flat articles such as sheeting, and with such devices it is not possible easily to change the type of article produced.